Testing

Provides a new network hashrate calculation method taking into account difficulty changes :

=> Previous implementation estimated hashrate by calculating hashrate (rate = D * 2^32 * N blocks/Time to find N blocks)
=> Now calculates expected number hashes for each block in sample using the following formula : rate = (sum[m = height to height - N step -1] D(m) * 2^32) / (T(height) - T(height - N)